1. Field of the Invention
The present invention relates to mechanical methods for operating satellites utilizing synthetic aperture radar and holography for imaging. The method of the present invention therefore primarily involves the application of principles in the fields of orbital mechanics, classical mechanics, and analytical geometry to achieve improvements in synthetic aperture radar methods. The method of satellite operation disclosed and claimed herein is particularly useful for detecting minerals in the ground, forest cover, agricultural growth starter, soil moisture states or other conditions in the illuminated area.
2. Description of the Prior Art
In the past, satellites have been used for various communication purposes. For example, in my prior U.S. Pat. No. 3,243,706, issued Mar. 29, 1966, there was disclosed a satellite net configured for uninterrupted global coverage and dynamically balanced tracking. That patent disclosed the use of three satellite sub-systems with four satellites employed in each sub-system, all being mutually perpendicular with two of the sub-systems in polar planes 90.degree. apart. The remaining sub-system was in an equatorial plane. The arc distance from plane crossings was nominally equal for all satellites. That system was directed primarily to communication satellite linking. It was also useful for certain types of imaging, but suffered from an inability to provide fine ground resolution. In order to facilitate communication functions, the method of U.S. Pat. No. 3,243,706 required that the satellites utilized be in high altitude orbits which necessarily resulted in poor ground resolution for optical imaging.
Another bistatic synthetic aperture radar method was disclosed in U.S. Pat. No. 4,325,065. This patent disclosed a method of producing range-doppler imagery which was a linear construct over a field of view limited by the aircraft altitude. U.S. Pat. No. 4,325,065 taught a flat-earth mathematical model, which was apropos only to small viewing areas visible from aircraft altitudes wherein the aircraft traversed a distance of less than 30 miles, or a great circle distance of less than one-half degree. Thus, the application of this method was severely limited for satellite operations since satellites operate at much greater altitudes and velocities than aircraft. Thus, this flat-earth model was unacceptable for satellite operations wherein a spherical-earth mathematical model was preferred to focus the nominally spherical surface of the earth for large fields of view, to provide large synthetic apertures for fine resolution and high contrast.
Walker in his article titled "Range-Doppler Imaging of Rotating Objects", IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-16, No. 1 January 1980 disclosed a range-doppler imaging radar which used a prediction model based upon a rotating spherical earth. Walker's method was perfectly linear, wherein a point target in the object field created a family of straight lines in the video recording on a rotating disc medium. In contrast to the method in U.S. Pat. No. 4,325,065, which required a known reference point in the field of view, and where the doppler history of the point was required to be known a priori, Walker's method only required a rotational history of the object as a whole as the a priori knowledge.
All of the prior art range-doppler imaging methods had various limitations and short comings which are overcome by the method of the present invention. For example, the method of U.S. Pat. No. 4,325,065 was useful for obtaining unregistered images for small fields of view, but not for mapping wherein cartographic registration to a geodetic datum is required. This method was also not feasible for broad ocean imaging wherein known reference points are usually not available. If this method were to be applied to satellites, the field of view would necessarily be restricted to an area of less than one thousand square miles in order to limit the errors imposed by the use of a flat-earth model. Therefore, a global mosaic of the small area images would require more than one-hundred thousand image plates. Since these plates would for the most part not have known geodetic reference points, as most of the earth's surface, i.e., the oceans, could not be imaged, then global geodetic control would not be practical. Furthermore, the imagery would not be made conformal since the known reference points would be distributed randomly whereas conformal plates must be distributed uniformly to enable equal projection at the overlap or match-line points between adjacent conformal plates.
Walker's method could only be adapted to provide an imaging system for bistatic synthetic aperture radar satellites if a method for providing conformal system mapping planes with system phase center intercepting the center of the rotating object field was devised. Such a system for conformal mapping of the synthetic aperture radar imagery is part of the method of the present invention, wherein the geometry and dynamics of the system are designed to accommodate linear range-doppler recording and reconstruction of bistatic synthetic aperture radar images on disc recording media, recorded as parallel to one of a conformal geodetic mapping set of eight planes encompassing the entire earth surface. The method of the present invention is therefore a significant improvement over these prior art methods.
All prior or synthetic aperture radar methods and range-doppler imaging radar suffered degraded images from an aberration known as range curvature. This aberration was caused by a failure in these methods to preserve signal surface orthogonality throughout the recording plane at all times. In the preferred method of the present invention, orthogonality of the signal surface is preserved at all points and at all times in the recording plane by the method whereby the bistatic synthetic aperture radar base-line is inertially fixed parallel to the recording plane. This elimination of range curvature is a significant improvement over all previous synthetic aperture radar range and range-doppler radar methods.